The present invention concerns optical fibre transmissions and more specifically a coupler for a multimode pump.
Recently, xe2x80x9cphotonicxe2x80x9d fibres have appeared on the market, these fibres known as xe2x80x9cphotonic crystal fibresxe2x80x9d (PCF). Like conventional fibres, these fibres are not fully constituted by a solid transparent material, such as doped silicon. Shown as a section, a photonic fibre exhibits a plurality of air holes. These holes are parallel to the axis of the fibre and extend longitudinally along the fibre. In practice, these holes can be obtained by producing the preform by assembling silicon cylinders or capillary tubes whilst observing the pattern of the holes to be obtained in the fibre. Stretching this preform provides a fibre with holes corresponding to the capillary tubes.
The presence of these holes in the material of the fibre creates average index variations of the material. As in a conventional optical fibre, these index variations can be used to guide light signals with adapted wavelengths. A description of these photonic fibres is provided in the document WO-A-00 49 435. This document, apart from showing the operating principle of the photonic fibres, also describes a directional coupler or ray separator. This coupler is formed by heating and stretching two photonic fibres. The stretched areas of the two photonic fibres are welded during heating or during a subsequent heating stage. The light propagating in one of the fibres is then coupled into another fibre. This document also proposes assembling a photonic fibre with a conventional fibre, that is a fibre without any holes, the photonic fibre then being stretched and coupled into the stretched zone. Stretching of the photonic fibre recloses the holes of the fibre so that there are no longer any holes close to the cut. The conventional fibrexe2x80x94according to the figure a cladded core fibrexe2x80x94is also stretched and cut so as to have a size identical to the size of the stretched photonic fibre. The two fibres are assembled end-to-end. The mode diameter extends over the entire fibre in the stretched areas in the shape of a cone. In the non-stretched portion of the photonic fibre or conventional fibre, the light propagates normally. Finally, this document indicates that the local stretching of a multi-core photonic fibre makes it possible to create a local directional coupler. In fact, the stretching of the fibre provokes a partial closing of the holes and an interaction between the lights of the various cores.
The document EP-A-1 043 816 describes a double clad fibre. The signal is transmitted into the doped core of the fibre and a pump is injected into the first cladding. So as to direct the light of the pump towards the doped core, regions with a modified index are provided in the first cladding. These modified index regions can in particular be constituted by air holes. Nothing in this document indicates how the pump is coupled in the cladding of the fibre.
The document EP-A-893 862 describes a bundle of fibres having six pump injection multimode fibres surrounding a monomode fibre for the signal. The fibres are assembled by melting and stretching so as to reduce the diameter of the six injection fibres and the monomode fibre to the size of the core of a double cladding fibre. The bundle obtained is cut and coupled end-to-end to the double cladding fibre. The injection fibres are multimode fibres having a digital opening smaller than that of the double cladding pumping fibre. The monomode fibre is a clad core fibre which is used for injection of the signal to be amplified in the double cladding fibre or for extracting the signal from this fibre. The mode diameter in the monomode fibre is a decreasing and then increasing function of the core diameter. Thus, it is possible to select for the monomode fibre a core diameter after melting and stretching, which ensures the same mode diameter at the inlet and outlet of the monomode fibre.
This solution limits the degrees of freedom concerning the design of the system. In fact, the core diameters at the two extremities of the coupler are linked. This can render production more complex. In addition, the solution requires that the mode diameter decreases and then increases when the stretched portion of the monomode fibre is traversed, which can cause problems. Finally, the monomode nature of the fibre for high values of the core diameter is not necessarily ensured.
With respect to the above information, the problem concerned by the invention is to simplify the production of this coupler and also simplify management of the optical characteristics of the coupler along the path followed by the signal.
More specifically, the invention concerns an optical coupler having:
a photonic optical fibre having a stretched portion;
at least one multimode optical fibre coupled to the photonic optical fibre.
In one embodiment, the photonic optical fibre is at the centre of assembling with the multimode optical fibres. The photonic optical fibre can be assembled with the multimode optical fibre(s) by melting, stretching or even be assembled with the multimode optical fibres outside the stretched portion.
The photonic fibre can also be an amplifying fibre. In one embodiment, the photonic fibre is a multimode fibre. It is then advantageous that it possesses in a non-stretched state a multimode core diameter greater than or equal to 100 micrometers. In another embodiment, the photonic fibre is a double cladding fibre. In this case, it is preferable that it has in a non-stretched state a multimode core diameter greater than or equal to 100 micrometers and/or a mode diameter for the monomode core larger than or equal to 15 micrometers.
The photonic optical fibre can also have a second stretched portion at its extremity opposite the stretched portion.
The invention also concerns a fibre laser having:
this coupler as described above with a multimode photonic fibre;
a laser cavity formed of an optical fibre connected to the stretched extremity of the photonic fibre of the coupler;
at least one multimode pump connected at the inlet of a multimode optical fibre of the coupler.
Advantageously, the difference between the diameter of the multimode core of the photonic fibre and the diameter of the multimode core of the laser cavity is less than or equal to 5%.
The invention further concerns an optical amplifier having:
the above-mentioned coupler with a double cladding photonic fibre
a multiple cladding amplifying optical fibre coupled to the stretched extremity of the coupler.
In this case, it is advantageous that the difference between the mode diameter of the monomode core of the multiple cladding optical fibre and the mode diameter of the monomode core of the photonic fibre is less than or equal to 5%. It is also possible to ensure that the difference between the diameter of the multimode core of the multiple cladding optical fibre and the diameter of the multimode core of the photonic fibre is less than or equal to 5%.
In one embodiment, the amplifier has a second coupler of the same type, the extremity of the multiple cladding fibre opposite the first coupler being coupled to the stretched extremity of the second coupler.